Bayonet tube heat exchange

ABSTRACT

A bayonet tube heat exchanger in which problems at the upper tube sheet created by high temperatures are eliminated. The bayonet tube assemblies are designed so that the heated secondary fluid which, having been heated in the space between the coaxial tubes in the lower portion of the bayonet tube assemblies, flows through the main tube sheet through a conduit which is inside of but spaced from the outermost of the coaxial tubes.

United States Patent 1191 Kissinger 1451 Jan. 21, 1975 BAYONET TUBE HEATEXCHANGE [75] Inventor: John A. Kissinger, Livingston, NJ.

[73] Assignee: Foster Wheeler Corporation,

Livingston, NJ.

22 Filed: Sept. 21, 1972 21 Appl, No.: 291,099

3,482,626 12/1969 Gainsboro et al. 165/141 X Primary Examiner-BenjaminW. Wyche Assistant Examiner-Theophil W. Streule Attorney, Agent, orFirm-John E. Wilson; Marvin A. Naigur [57] ABSTRACT A bayonet tube heatexchanger in which problems at the upper tube sheet created by hightemperatures are eliminated. The bayonet tube assemblies are designed sothat the heated secondary fluid which, having been heated in the spacebetween the coaxial tubes in the lower portion of the bayonet tubeassemblies, flows through the main tube sheet through a conduit which isinside of but spaced from the outermost of the coaxial tubes.

4 Claims, 2 Drawing Figures BAYONET TUBE HEAT EXCHANGE BACKGROUND OF THEINVENTION One type of heat exchanger which is presently being used inindustry is the bayonet tube type. In the bayonet tube heat exchanger,the bayonet tube assemblies which carry the secondary fluid intoindirect heat exchange with the primary fluid comprise two coaxialtubes. The outer tube has a closed end and the secondary fluid is flowedthrough the inner tube until it reaches the end of the bayonet tubeassembly where it impinges against the closed end of the outer tube toreverse its direction and flow back in the annular space between theinner and outer tubes. It is when the secondary fluid flows between theinner and outer tubes that it picks up the greater part of the heatwhich is transferred to it by the primary fluid. The secondary fluid iscollected between the tube sheet on which the outer tubes are mountedand another tube sheet which is connected to the ends of the innertubes. There is a space between the tube sheets because the inner tubesare longer than the outer tubes.

In heat exchangers which operate at a high temperature, the temperatureof the tube sheet to which the outer tubes are mounted will be very highbecause the secondary fluid when flowing through the tube sheet, hasreceived all of the heat it will receive from the primary fluid.

A practical example of such a heat exchanger is one which receives heatfrom a primary fluid which is lithium, molten salt or liquid sodium.Such a heat exchanger is likely to have vertically extending bayonettubes so that the main tube sheet which supports the outer tube islikely to be lower than the one which supports the inner tubes and iscloser to the primary fluid.

High temperatures in the area of the connection between the outer tubesand the main tube sheet can be problematic where there is an abruptdiscontinuation of the flow of liquid sodium coming into the heatexchanger. The temperature in the critical area will change within avery short time. When the temperature drops over a large range abruptly,components which are affected by the change can fail. For instance, ifthe main tube sheet should suddenly cool, it could become detached fromthe outer tube because of shrinkage.

Even without abrupt temperature drops, high temperatures in the area ofthe outer tube and main tube sheet are undesirable because of limitationon the life of the adjacent components due to creep. Changes in therelative dimensions of components in the area of the main tube sheeteven when gradual are undesirable because tight fits are necessary forsafe and efficient operation.

SUMMARY OF THE INVENTION It is an object of the present invention toovercome drawbacks found in the prior art, such as those discussedabove. Accordingly, a bayonet tube type heat exchanger having bayonettube assemblies, each with an outer tube closed at one end and fitted amain tube sheet at the other end and a coaxial inner tube open at bothends, is provided with means to flow the heated secondary fluid throughthe main tube sheet through a conduit inside of and spaced from theouter tube.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a view partly in sectionshowing a bayonet tube made in accordance with the present invention;and

FIG. 2 is a view in section substantially along the line 22 of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWING The drawing shows a bayonet tubeassembly 10 which is connected at its upper end 12 to a downwardlyprojecting spigot 14 on the lower side of a main tube sheet 16.

The bayonet tube assembly 10 is one of many. each of which is attachedto a spigot like the one I4 but only one is shown and described indetail since they are all similar in design and function. The bayonettube assembly 10 is heated by a primary fluid which flows over it andeach of the other bayonet tubes.

The secondary fluid, that is the fluid to be heated, flows down throughthe bayonet tube assembly 10 through an inner tube 18 which may be ofdouble wall construction over most of the its length, as shown. Thefluid flows down through the inner tube 18 until it reaches the end ofthe tube. The bayonet tube assembly includes an outer tube 20 which isclosed at its lower end 22 and the inner tube 18 is positioned withinthe outer tube 20 so that after the secondary fluid has flowed throughthe inner tube 18, it impinges against the lower end 22 of the outertube 20 to reverse its direction and flow upwardly in the annular spacebetween the inner l8 and the outer tube 20. The double wall constructionof the inner tube 18 prevents the transfer of heat from the secondaryfluid flowing upwardly between the inner tube 18 and the outer tube 20and the cooler secondary fluid flowing downwardly through the inner tube18.

In order to prevent the transfer of heat from the hot secondary fluidflowing upwardly in the bayonet tube assembly 10 to the tube sheet 16,an inner conduit 26 is provided. The inner conduit 26 has a lower end 28which is located at a level below the main tube sheet 16 so thatsecondary fluid flowing upwardly between the inner tube 18 and the outertube 20 will enter the inner conduit 26 and flow through that conduit asthe secondary fluid passes through the main tube sheet 16. As shown inFIG. 2, the inner conduit 26 has a somewhat moon-shaped cross-section sothat the available space in the outer tube 20 can be used effectively.The inner conduit 26 extends above the tube sheet 16 and is connected toa header pipe 30 which extends horizontally and which connects withother inner conduits to pick up hot secondary fluid coming out of thevarious bayonet tube assemblies in the heat exchanger. The header pipe30 is connected with one or more feeder pipes 32, which pass through asecondary tube sheet 34.

The inner tube 18 extends centrally through the outer tube 20 over mostof its length but in the portion of the bayonet tube 20 between thelower end 28 of the inner conduit 26 and the top of the main tube sheet16 is set off to the side of the bayonet tube assembly as shown in FIG.2.

The inner tube 18 and inner conduit 26 are surrounded with stagnantsecondary fluid. The fluid does not move because the space between themain tube sheet 16 and the secondary tube sheet 34 is sealed as by wall35.

The inner tube 18 is connected to a header pipe 36 which supplies fluidto other inner tubes. The header pipes 36 are fed fluid through thefeeder pipes 38 which pass through the secondary tube sheet 34.

In order to keep the secondary fluid which has not been heated and whichmust pass through the inner tubes 18 separated from the secondary fluidwhich has already been heated, a division wall 40 is provided. Thefeeder pipes 38 communicate with the space above the secondary tubesheet 34 to one side of the division wall 40 whereas the feeder pipes 32communicate with the space above the tube sheet 34 to the other side ofthe division wall 40.

With the present invention, there will be no excessive heating of thetube sheet 16 because the heated secondary fluid will not transfer anyconsiderable amount of heat to the tube sheet because it passes throughthe inner conduit 26 which does not contact the outer tube 20 in thevicinity of the tube sheet 16. Thus, the tube sheet 16 will not beweakened because of high temperatures and a sudden discontinuance of hotprimary fluid resulting in a sudden drop in the temperature of thesecondary fluid passing upwardly from the bayonet tube assembly will notresult in shock to the upper tube sheet 16 because the secondary fluiddoes not transfer heat to any great extent to the tube sheet 16 as itpasses through it.

The foregoing describes but one preferred embodiment of the presentinvention, other embodiments being possible without exceeding the scopeof the present invention as defined in the following claims.

What is claimed is:

l. A bayonet tube assembly comprising:

an outer tube being a closed end and an open end adapted to be connectedto a tube sheet;

an inner tube within said outer tube, having an open end adjacent tosaid closed end of said outer tube, and being longer than said outertube so that when said outer tube is connected to said tube sheet, saidinner tube will extend through said tube sheet;

an inner conduit within the space between said inner tube and said outertube and located to extend from a location between said tube sheet andsaid open end of said inner tube through said tube sheet wherebysecondary fluid can be passed through said inner tube to reverse itsdirection and flow back between said inner tube and said outer tubeuntil reaching said inner conduit and then flow through said conduit topass through said tube sheet.

2. The bayonet tube assembly defined in claim 1 wherein said innerconduit is moon-shaped in crosssection.

3. A bayonet tube-type heat exchanger wherein secondary fluid flowingthrough a bayonet tube is heated by primary fluid flowing over said tubecomprising:

a main tube sheet;

an outer tube having a closed end and an open end,

said outer tube being connected to said tube sheet at said open end;

an inner tube within said outer tube, said inner tube having an open endadjacent to said closed end of said outer tube and extending throughsaid tube sheet;

an inner conduit, of a length considerably less than said inner tube orsaid outer tube, within the space between said inner tube and said outertube and extending through said main tube sheet;

the space within said outer tube not enclosed by said inner tube andsaid inner conduit being sealed to create a stagnant layer of secondaryfluid when secondary fluid is flowed down said inner tube toward saidclosed end of said outer tube to reverse its direction and flow backbetween the inner and outer tubes and then through said inner conduitand said main tube sheet.

4. The bayonet tube heat exchanger defined in claim 3 further comprisinga secondary tube sheet, a division wall separating the space above saidsecondary tube sheet into two chambers, the inner tubes communicatingwith one of said chambers, the inner conduit communicating with theother of said chambers.

1. A bayonet tube assembly comprising: an outer tube being a closed endand an open end adapted to be connected to a tube sheet; an inner tubewithin said outer tube, having an open end adjacent to said closed endof said outer tube, and being longer than said outer tube so that whensaid outer tube is connected to said tube sheet, said inner tube willextend through said tube sheet; an inner conduit within the spacebetween said inner tube and said outer tube and located to extend from alocation between said tube sheet and said open end of said inner tubethrough said tube sheet whereby secondary fluid can be passed throughsaid inner tube to reverse its direction and flow back between saidinner tube and said outer tube until reaching said inner conduit andthen flow through said conduit to pass through said tube sheet.
 2. Thebayonet tube assembly defined in claim 1 wherein said inner conduit ismoon-shaped in cross-section.
 3. A bayonet tube-type heat exchangerwherein secondary fluid flowing through a bayonet tube is heated byprimary fluid flowing over said tube comprising: a main tube sheet; anouter tube having a closed end and an open end, said outer tube beingconnected to said tube sheet at said open end; an inner tube within saidouter tube, said inner tube having an open end adjacent to said closedend of said outer tube and extending through said tube sheet; an innerconduit, of a length considerably less than said inner tube or saidouter tube, within the space between said inner tube and said outer tubeand extending through said main tube sheet; the space within said outertube not enclosed by said inner tube and said inner conduit being sealedto create a stagnant layer of secondary fluid when secondary fluid isflowed down said inner tube toward said closed end of said outer tube toreverse its direction and flow back between the inner and outer tubesand then through said inner conduit and said main tube sheet.
 4. Thebayonet tube heat exchanger defined in claim 3 further comprising asecondary tube sheet, a division wall separating the space above saidsecondary tube sheet into two chambers, the inner tubes communicatingwith one of said chambers, the inner conduit communicating with theother of said chambers.